Selenium is a trace element currently recognized to be highly toxic as well as nutritionally essential. Consequences of long-term Se intake and the possible adverse health effects of some Se compounds remain in dispute. In order to understand the dual nature of Se and its unique cellular metabolism, at the level most likely to affect its role in the food chain, our overall objective will be to explain and compare the assimilation of this element by Se-sensitive and Se-utilizing plants and microorganisms. We propose to undertake the following investigations: 1. Examine the exclusion of selenocysteine and selenomethionine from proteins of certain members of the genus Astragalus, plants able to accumulate high levels of Se. Callus cultures of these plants will be established and used as source material to investigate the exclusion mechanism at the subcellular level; comparisons will be made with callus cultures from Se-sensitive non-accumulators. 2. Study the metabolism of Se in Baccilus sp. strain SS, our newly isolated aerobic spre-former that not only grows in the presence of selenite at concentrations as high as 100mM, but also, in certain media, requires the element or its growth: we will determine if the transport system is independent of the one for sulfate or selenate, investigate the incorporation of Se into proteins, locate the subcellular distribution of Se by means of electron microscopy techniques, and isolate other Se-utilizing microorganisms associated with seleniferous plants and soils. Newly isolated microorganisms will be further identified by biochemical characterization and computer-assisted tests at the American Type Culture Collection. 3. Examine the mode of entry of Se into formate dehydrogenase, the selenoenzyme synthesized by E. coli, to determin if selenocysteine is incorporated as such during formation of this protein or if it is synthesized by means of a post-translational madification of a polypeptide chain. Isoelectric focusing and electrophoresis will be used for these determinations.